It is known in the art to measure engine fuel economy based on average vehicle speed and the average fuel flow over an interval of time. This type of measured fuel economy considers fuel energy to be the only type of energy associated with the vehicle. In reality, the vehicle can also store or consume kinetic and/or potential energy. The chemical energy stored in the fuel may be converted into kinetic energy which may be used later during a coast down.
In vehicles with deceleration fuel shutoff, prior art methods of determining fuel economy indicate that fuel economy is infinite during deceleration. However, the fuel energy stored as kinetic energy during the acceleration phase is now being consumed. Furthermore, the prior art does not distinguish between braking and coasting, both of which may indicate infinite fuel efficiency.
A method and apparatus for calculating corrected vehicle fuel economy is disclosed in U.S. Pat. No. 4,845,630, issued to Stephens. The method includes calculating the distance travelled by the vehicle, the fuel consumed by the engine, and the change of the kinetic energy of the vehicle. The change of kinetic energy is compensated by a weighting factor that modifies the kinetic energy term to account for the fuel that would be used at real engine and drive train efficiencies, which can only be determined experimentally. Therefore, the weighting factor must be predetermined and stored in a read-only memory. This system does not to consider the potential energy of the vehicle.
The known prior art does not disclose a method of calculating fuel economy which balances the chemical energy contained in the fuel, and the kinetic and potential energy of the vehicle. In the prior art, a weighting factor is employed which takes into account that in converting chemical energy of the fuel into the shaft output, the efficiency is usually less than 25%. In comparison, potential energy can be converted into kinetic energy at a much higher efficiency. Therefore, the present invention calculates fuel economy based on an energy balance over the vehicle as a whole.